Essential Oil Composition of Grindelia squarrosa from Southern Idaho

Grindelia squarrosa is an arid lands herb that has been used in Native American traditional medicine, is a potential source of pharmacologically active compounds, and has been explored as a source of biofuel. The purpose of this work was to examine the essential oil composition of G. squarrosa from southern Idaho. Gas chromatographic methods revealed the essential oil of G. squarrosa var. serrulata to be rich in monoterpenoids, α-pinene (21.9%), limonene (17.1%), terpinolene (10.6%), and borneol (6.5%). The essential oil composition of G. squarrosa from Idaho is similar to that previously reported from specimens collected from Montana and confirms the volatile phytochemistry of plants growing in North America. The major essential oil components were screened for antimicrobial activity against respiratory and dermal pathogens. (–)-β-Pinene showed strong antibacterial activity against Streptococcus pneumoniae (MIC 39.1 μg/mL) and (–)-borneol showed strong activity against Staphylococcus aureus (MIC 78.1 μg/mL).


Introduction
The genus Grindelia Willd. (Asteraceae) is made up of around 73 species [1], naturally ranging in western and central North America, Mexico, and South America, but introduced to eastern North America and the Old World [2]. Grindelia squarrosa (Pursh) Dunal (Asteraceae), curlycup gumweed, is a short-lived herb or subshrub, with oblong to oblanceolate, crenulate-serrulate leaves (1.5-7 cm long and 0.5-1.3 cm wide), and ranges throughout the Great Plains, Rocky Mountains, and Great Basin areas of North America. The plant has been introduced elsewhere [3,4], and has become an invasive species in central Europe [5]. Several varieties of G. squarrosa have been described [6], and three have been recorded in Idaho, namely G. squarrosa var. quasiperennis Lunell, G. squarrosa var. serrulata Steyerm., and G. squarrosa var. squarrosa Cronquist [7].
The Shoshoni Native Americans used G. squarrosa in their traditional medicine. A decoction of the plant was used as a cough medicine, as an antiseptic wash, and taken internally as an expectorant, to treat stomachache, smallpox, and measles [8]. Labdane diterpenoids (e.g., grindelic acid, 6-oxogrindelic acid, and 17-hydroxygrindelic acid) have been isolated and identified from G. squarrosa [9] and the plant has been investigated as a source of biofuel [10]. In this work, we hypothesize that the essential oil may contain components supporting the Native American use of the plant; we present the essential oil composition of the aerial parts of G. squarrosa var. serrulata collected from southern Idaho and antimicrobial screening of some major essential oil components.

Results and Discussion
Based on botanical descriptions [11] and comparison with specimens from the New York Botanical Garden [7] the plant was identified as G. squarrosa var. serrulata (see Figure 1).
The Native American traditional medicinal use of the plant as a cough medicine and as an antiseptic wash prompted investigation of the antimicrobial activities of the major essential oil components. The compounds α-pinene, β-pinene, limonene, borneol, and bornyl acetate were screened for antimicrobial activity against the respiratory and dermal pathogenic bacteria Cutibacterium acnes, Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, Streptococcus pyogenes, and the dermatophytic fungi Microsporum canis, Microsporum gypseum, Serratia marcescens, Trichophyton mentagrophytes, and Trichophyton rubrum (Table 4).   Based on previously published guidelines [20,21], essential oil components showing MIC values < 500 µg/mL should be considered as showing "strong activity". Thus, S. pneumoniae was the most susceptible microorganism and S. pyogenes was the most resistant to the essential oil components. Notably, the major enantiomers, (-)-α-pinene, (-)-β-pinene, (+)-limonene, (-)-borneol, and (-)-bornyl acetate, generally showed broad antimicrobial activity. Furthermore, (±)-α-pinene and (-)-β-pinene have shown strong activity against methicillin-resistant S. aureus (MRSA) with IC 50 values of 68.6 and 51.4 µg/mL, respectively [22], and both α-pinene and β-pinene (enantiomers not indicated) were active against Klebsiella pneumoniae with MIC values of 178 µg/mL and 170 µg/mL, respectively [23]. Terpinolene, not available for screening in this study, was found to be inactive (MIC >> 2000 µg/mL) against C. acnes and S. aureus [24]. Schepetkin and co-workers found that G. squarrosa essential oil as well as (−)-borneol activated human neutrophils [19]. Neutrophils play a critical role in inflammation. The antimicrobial activities of G. squarrosa essential oil components, coupled with the modulation of human neutrophil function of (−)-borneol, are consistent with Native American use of G. squarrosa to treat respiratory and dermal conditions.

Plant Material
Aerial parts of Grindelia squarrosa were collected from several plants growing wild near Bogus Basin Ski Resort, Idaho on July 7, 2022 (43 • 43 34 N, 116 • 9 28 W, 1482 m elevation). The plant was identified by W.N. Setzer. Based on botanical descriptions [11] and comparison with specimens from the New York Botanical Garden [7] the plant was identified as G. squarrosa var. serrulata. A voucher specimen (WNS-Gss-5718) has been deposited in the University of Alabama in Huntsville herbarium. The fresh plant material from several plants was combined and 91.11 g was hydrodistilled to give 597 mg of a colorless essential oil.

Gas Chromatographic Analysis
The essential oil of G. squarrosa var. serrulata was analyzed via gas chromatography mass spectrometry (GC-MS), gas chromatography with flame ionization detection (GC-FID), and chiral GC-MS as previously described in [25]. Briefly, gas chromatography-mass spectrometry (GC-MS) was carried out using a Shimadzu GC-MS-QP2010 Ultra (Shimadzu Scientific Instruments, Columbia, MD, USA). The mass selective detector was operated in the electron impact (EI) mode with an electron energy of 70 eV, a scan range of 40-400 atomic mass units and a scan rate of 3.0 scans per second, using the GC-MS solution software. The GC column used was a Zebron ZB-5ms fused silica capillary column (Phenomenex, Torrance, CA, USA), 60 m in length and 0.25 mm inner diameter; the stationary phase was (5% phenyl)-polydimethylsiloxane with a film thickness of 0.25 µm. The carrier gas was helium and the column head pressure was 208.3 kPa with a flow rate of 2.00 mL/min. The injector temperature was 260 • C, the interface temperature was 260 • C, and the ion source temperature was 260 • C. The GC oven temperature was programmed with an initial temperature of 50 • C; the temperature was increased to 260 • C at a rate of 2 • C/min, and then held at 260 • C for 5 min, for a total GC acquisition time of 110 min. The solvent cut time was set at 5 min. A 5% (w/v) solution of G. squarrosa var. squarrosa essential oil in dichloromethane was prepared and a volume of 0.1 µL was injected and the splitting mode was set at 24.5:1. Retention index (RI) values were determined by calibrating the instrument using a homologous series of nalkanes using the logarithm-based arithmetic index method developed by van den Dool and Kratz [26]. The components of the essential oil were identified by comparing the mass spectral fragmentation patterns and the retention index values available in the Adams [27], FFNSC3 [28], NIST20 [29], and Satyal [30] databases.
Gas chromatography with flame ionization detection (GC-FID) was carried out on G. squarrosa var. squarrosa essential oil using a Shimadzu GC 2010 equipped with a flame ionization detector (Shimadzu Scientific Instruments, Columbia, MD, USA) and using a Zebron ZB-5 GC column (60 m × 0.25 mm × 0.25 µm film thickness) (Phenomenex, Torrance, CA, USA). The same operating conditions were used for the GC-FID as those for GC-MS (above). The percent compositions were calculated from raw peak areas without standardization.
The G. squarrosa var. squarrosa essential oil was analyzed via chiral gas chromatographymass spectrometry using a Shimadzu GCMS-QP2010S instrument (Shimadzu Scientific Instruments, Columbia, MD, USA). The mass selective detector was operated in the electron impact (EI) mode with an electron energy of 70 eV, a scan range of 40-400 atomic mass units and a scan rate of 3.0 scans per second that was fitted with a Restek B-Dex 325 chiral GC column (30 m length × 0.25 mm inner diameter × 0.25 µm film thickness) (Restek Corp., Bellefonte, PA, USA); the stationary phase was 25% 2,3-di-O-methyl-6-O-t-butyldimethylsilyl-β-cyclodextrin in SPB-20 poly(20% phenyl/80% dimethylsiloxane) phase with a film thickness of 0.25 µm. Helium was the carrier gas, the column head pressure was 53.6 kPa, and the flow rate was 1.00 mL/min. The injector temperature was 240 • C, the ion source temperature was 240 • C, and the interface temperature was 240 • C. The solvent cut time was 5 min. The GC oven temperature was programmed with an initial temperature of 50 • C, which was held for 5 min, then increased at a rate of 1 • C/min until a temperature of 100 • C, after which the temperature was increased at a rate of 2 • C/min to 220 • C, for a total GC acquisition time of 107 min. A 0.3-mL sample of the essential oil (5% w/v in dichloromethane) was injected using a splitting mode of 24.0:1. The compound enantiomers were determined by comparing their retention times with authentic samples obtained from Sigma-Aldrich (St. Louis, MO, USA). The ratios of enantiomers were calculated from raw peak areas.

Antibacterial and Antifungal Screening
The essential oil components, (+)-α-pinene, ( [31,32]. Each of the bacterial strains was cultured on tryptic soy agar medium. A 5000-µg/mL solution of each test compound was prepared in dimethylsulfoxide (DMSO, Sigma-Aldrich, St. Louis, MO, USA), and 50 µL was diluted in 50 µL of cation-adjusted Mueller Hinton broth (CAMBH) (Sigma-Aldrich, St. Louis, MO, USA), the 100-µL mixture was added to the top well of a 96-well microdilution plate. The prepared stock solution of each compound was serially two-fold-diluted in fresh CAMBH to obtain final concentrations of 2500, 1250, 625, 312.5, 156.3, 78.1, 39.1, and 19.5 µg/mL (final DMSO concentrations of 50%, 25%, 12.5%, 6.25%, 3.13%, 1.56%, 0.78%, and 0.39%). Freshly harvested bacteria with approximately 1.5 × 10 8 colony-forming units (CFU) per mL final concentration (determined using McFarland standard) were added to each well of the 96-well microdilution plates, which were then incubated at 37 • C for 24 h. Gentamicin (Sigma-Aldrich, St. Louis, MO, USA) was used as the positive antibacterial control and DMSO was the negative control. The minimum inhibitory concentration (MIC) was determined to be the lowest-concentration well that did not show turbidity. Each assay was carried out in triplicate.
For the antifungal screening, the tested fungi were cultured on yeast malt agar (Sigma-Aldrich, St. Louis, MO, USA). Stock solutions (5000 µg/mL) of the test compounds were prepared in DMSO and diluted as above in fresh yeast-nitrogen growth medium (broth) (Sigma-Aldrich, St. Louis, MO, USA). The freshly harvested fungi, with approximately 7.5 × 10 7 CFU/mL final concentrations in yeast-nitrogen growth medium, were added to each well of the 96-well microdilution plates and were then incubated at 35 • C for 24 h. Amphotericin B (Sigma-Aldrich, St. Louis, MO, USA) served as the positive antifungal control, while the negative control was DMSO. The antifungal assays were carried out in triplicate.

Conclusions
This is the first report on the essential oil characterization of Grindelia squarrosa var. serrulata from southern Idaho. The essential oil was rich in monoterpenoids and comparable in composition to G. squarrosa (variety not indicated) from western Montana, which suggests chemotype stability in North American populations; however, it was very different from G. squarrosa essential oils cultivated in Europe. The antimicrobial activities of the major components of G. squarrosa essential oil support the use of the plant to treat respiratory and dermal infections.